Methods for extending the life of alloy steel welded joints by elimination and reduction of the HAZ

ABSTRACT

In one embodiment, the present invention provides a method for welding together two metal pieces, comprising buttering a surface of a first metal piece with a first nickel-based filler metal at a thickness sufficient to isolate a heat-affected zone in the first metal piece from subsequent welding; heat-treating at least the heat-affected zone in the first metal piece; buttering a surface of a second metal piece with a second nickel-based filler metal having the same composition as the first nickel-based filler metal and at a thickness sufficient to isolate a heat-affected zone in the second metal piece from subsequent welding; heat-treating at least the heat-affected zone in the second metal piece; and welding the heat-treated first buttered surface to the heat-treated second buttered surface with a third nickel-based filler metal having the same composition as the first and second nickel-based filler metals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to welding. More specifically, theinvention is directed to a method of welding metal pieces together usingbuttering and heat treating techniques to avoid post-weld heat treatmentand to reduce or eliminate the heat-affected zone.

2. Description of Related Art

There are two aspects of welding that increase costs of welding and thatlead to failure of welded components: the presence of a heat-affectedzone (HAZ) and post-weld heat treatment (PWHT), which is used to addressthe problems presented by the HAZ. As is known in the art, the heat froma weld creates a HAZ in the metal adjacent to the weld. The creation ofthis HAZ has adverse metallurgical effects, such as the creation ofnotch effects or grain growth, which cause a weakening of the metal inthe HAZ. While new alloys, for example, 2-12% chromium content byweight, have been developed to provide higher strengths for hightemperature pressure applications compared to alloys and steelspreviously used, failures that limit the useful life of componentsproduced from these materials tend to occur next to welds in the HAZ. Inaddition, attempts to develop filler materials to strengthen welds andreduce the effects of the HAZ have not been satisfactory.

Another method used to improve the metallurgical properties in the HAZis a PWHT. The American Society of Mechanical Engineers (ASME) Boilerand Pressure Vessel Code requires that welding applications of low alloysteel piping and pressure vessels be subjected to a PWHT to achievetoughness, tensile and hardness properties in the weld HAZ. However,PWHT tends to be an expensive process requiring considerable time toperform. PWHT typically requires heating the welded metal piece to atemperature that is just below the first transformation temperature forthe material. ASME-mandated PWHT of a high temperature materialcontaining 2% chromium requires the material to be held at 1350° F. forone hour for each inch of material thickness. The ramp rate at which thetemperature of the material can be raised to the holding temperature andcooled back to room temperature must be closely controlled and requiresseveral hours to complete. Typical PWHT operations on a 2″ thick pipemanufactured from this material, including set up, time to raise thematerial to temperature, hold time, and cooling time can require 24hours. Components with multiple welds may require several PWHToperations.

Fabricators of components generally perform such PWHT in a large oventhat allows for several weld joints to be tempered concurrently. Thephysical size of the oven obviously limits the size of the componentsthat can be tempered. Therefore, some PWHT procedures must be performedin the field or at the job site. In these cases, parts are weldedtogether and then transferred to the field where a PWHT is performed.PWHT in such larger components are generally performed utilizingresistance pads or induction heat treatment equipment. As such, thenumber of weld joints that can receive PWHT at one time is limited bythe availability of power and PWHT equipment. On large constructionjobs, scheduling of PWHT also becomes an important task.

Moreover, simply moving components in the fabrication shop to the oven,storage, other fabrications areas or to the job site can result infailures in welds and HAZ areas. Assembly of components at the job sitepresents additional challenges to achieve acceptable PWHT. Air flowcurrents around the components, including, for example, both externaland internal currents from wind and chimney effects, can result in thematerial not reaching sufficient temperature during PWHT to develop therequired properties. Special care to support the components is alsorequired during PWHT operations since the strength of the material isgreatly reduced by the high temperature of the PWHT operation.

Even with PWHT, failures still occur near the weld or in the HAZ.Therefore, a need exists for an improved method of welding that reducesthe effects of the HAZ and eliminates the need for PWHT.

SUMMARY OF THE INVENTION

The present invention provides a method for welding together two metalpieces, comprising buttering a surface of a first metal piece with afirst nickel-based filler metal at a thickness sufficient to isolate aheat-affected zone in the first metal piece from subsequent welding toproduce a first buttered surface; heat-treating at least theheat-affected zone in the first metal piece after buttering of thesurface of the first metal piece to produce a heat-treated firstbuttered surface; buttering a surface of a second metal piece with asecond nickel-based filler metal having the same composition as thefirst nickel-based filler metal and at a thickness sufficient to isolatea heat-affected zone in the second metal piece from subsequent weldingto produce a second buttered surface; heat-treating at least theheat-affected zone in the second metal piece after buttering of thesurface of the second metal piece to produce a heat-treated secondbuttered surface; and welding the heat-treated first buttered surface tothe heat-treated second buttered surface with a third nickel-basedfiller metal having the same composition as the first and secondnickel-based filler metals.

The method of the present invention may be used to weld both similar anddissimilar metals. Preferably, the method of the present invention isused to weld a martensitic stainless steel to either a ferriticstainless steel, an austenitic stainless steel or another martensiticstainless steel

The method of the present invention allows for sub-component pieces tobe welded together either in the shop or at a job site without the needfor post-weld heat treatment. This provides for better utilization ofequipment and improved manpower scheduling while reducing the amount oftime required for field assembly. The methods of the invention describedherein can be utilized to substantially reduce the costs and timerequired to join low alloy piping and/or pressure vessel materials andis applicable to a variety of different welding processes.

These and other features and benefits of the invention will appear fromthe following description from which the preferred embodiments are setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram according to one embodiment of thepresent invention; and

FIGS. 2A-2C illustrate the welding of two metal pieces according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved method for welding thatreduces or eliminates the effects caused by the formation of the HAZduring welding and eliminates the need for PWHT. Generally, the presentinvention provides a method for preparing a first metal piece to bewelded using a buttering technique with a nickel-based filler materialfollowed by a PWHT that either tempers the HAZ or eliminates the HAZthrough normalization. A second metal piece is also prepared in asimilar or identical fashion. The two pieces are then welded together,either in the shop or in the field, without the need for PWHT.

FIG. 1 is a process flow diagram according to one embodiment of thepresent invention. The process 100 is a method for welding two metalpieces together. It should be appreciated that the metal pieces to bewelded together may be similar or dissimilar metals. For example, lowalloy ferritic steels, which have less than 12% chromium by weight, maybe joined together. Also, a low alloy steel having less than 12%chromium by weight, such as a ferritic steel, may be joined to astainless steel having 12% or more chromium by weight, such as anaustenitic stainless steel. In a preferred embodiment, the presentinvention is used to join a martensitic stainless steel to either aferritic stainless steel, an austenitic stainless steel or anothermartensitic stainless steel. Another preferred embodiment includes thejoining of 9Cr alloys to other 9Cr alloys, other ferritic alloys, oraustenitic alloys, where 9Cr alloys may include, for example, straight9Cr, P91, P92, etc.

It should be appreciated that the use of the term “low alloy” steelrefers to a steel similar to carbon steel, which are steels having lessthan approximately 1.65% manganese, 0.60% silicon or 0.60% copper,except that elements such as chromium, molybdenum, cobalt, columbium,titanium, etc. have been added to increase its hardenability andstrength. Examples of low alloy steels include 1¼ Cr-½Mo (T11 or P11),2¼ Cr-1 Mo (T22 or P22) and a variety of other ASTM-type alloy steelssuch as ferritic and martensitic steels. The use of the term “ferritic”refers to steels that exhibit a predominately ferritic microstructure atroom temperature and are not hardenable with heat-treatments. The use ofthe term “stainless” refers to ferrous alloys that contain a minimum of10% aluminum by weight and includes, for example, austenitic stainlesssteel, ferritic stainless steel, martensitic stainless steel andprecipitation-hardened stainless steel. The use of the term “austenitic”stainless steel refers to a stainless steel alloyed with nickel ormanganese and nitrogen to provide an austenitic structure at roomtemperature, such as the 300-series stainless steels, including, forexample, 304, 316, 321 and 347. The use of the term “martensitic”stainless steel refers to a stainless steel with the addition of carbonthat exhibits a predominantly martensitic microstructure at roomtemperature and is hardenable by heat treatment, such as the 400-seriesstainless steels, including, for example, 410, 420 and 440. Ferriticstainless steels, such as a 430 or 446 steel, contain a minimum of 10%chromium by weight and has a room temperature microstructure of ferriteand carbide. Typically, these alloys do not harden by heat treatment.

In the first step 102, each of the metal pieces to be welded together isprepared for welding. It should be appreciated that reference to a“metal piece” refers to any metal piece to be welded. For example, ametal piece may be a sub-component part that is to be welded to anothersub-component part to form a desired final component or piece ofequipment. Therefore, the use of the term “metal piece” is intended togenerically cover any type or shape of metal to be welded. Thepreparation done in this step 102, may include, various process steps orprocedures performed on a metal piece to prepare it for welding. Forexample, the metal piece, or the particular surface of the metal piece,may be machined into a particular shape by using a lathe. The surface ofthe metal piece may also be ground, which is often performed afterarc-gouging, or air-arcing, to remove metal. Electro-discharge machiningmay also be used for precision work; however, the process is typicallyslow.

In the next step 104, each surface of each metal piece to besubsequently welded to another metal piece is buttered using anickel-based filler. Specifically, a nickel-based filler is welded tothe particular surface of each piece of metal that will later be weldedtogether. The application of this nickel-based filler to the surface ofeach metal piece may be referred to as a “butter layer.” Preferably, thenickel-based filler comprises at least 10% or more nickel by weight andmore preferably comprises approximately 40-70% nickel by weight, andeven more preferably 40-60% nickel by weight. Otherwise, the filler maybe any material known in the art, which will depend, in part, on thecomposition of the metal pieces being welded together. Examples of somepreferred filler materials capable of being used in the presentinvention for buttering and welding of dissimilar metals include INCONELWelding Electrode 182, INCONEL Filler Material 82 and INCO-WELD AElectrode. It should be appreciated that preferably, the composition ofthe nickel-based filler is the same for both of the metal pieces or thebutter layers on each.

The actual conditions and techniques that may be used to apply or weldthis butter layer are well known to those of skill in the art.Specifically, any welding procedures or techniques currently availablemay be used to apply the butter layer. However, the butter layer shouldbe applied at a thickness sufficient to isolate the HAZ formed duringbuttering from heat produced during subsequent welding. In other words,the butter layer should be thick enough so that the HAZ is not affectedby subsequent welding operations, i.e., upon welding of the two metalpieces together.

Additionally, in the step 104, after application of the butter layer,each metal piece may again be machined to the appropriate weld geometryin preparation for final welding between the two metal pieces.Therefore, if such machining is performed, the thickness of the butterlayer should be sufficient to allow for a reduction in its thickness asa result of such machining. The thickness of the butter layer may varydepending on which type of welding process, the filler composition andthe alloy composition is used.

In the next step 106, each metal piece is heat-treated. The particularheat treatment applied should be sufficient to heat-treat at least theHAZ in each metal piece. In other words, each metal piece should besubject to heat such that at least the HAZ in each is heated to thedesired temperature to achieve the desired affect. In one embodiment,the heat treatment comprises heating each metal piece, or at least itsHAZ, to a temperature sufficient to normalize the HAZ. For example, inone embodiment, such heat treatment comprises heating each metal pieceabove the A_(C3) transformation temperature, which varies depending onthe alloy and its corresponding chemistry. The normalization of the HAZresults in restoration of the HAZ to its virgin or original basemetallurgical condition. Preferably, such heat treatment would beperformed in an oven to obtain better control than other heating methodsor equipment.

It should be appreciated that such normalizing heat treatment may beutilized in those cases where the metal pieces are both low alloy metalshaving a chromium content of approximately 2-12% by weight. Suchnormalizing heat treatment may be utilized each of the metal piecescomprises a low alloy ferritic steel piece. Such normalizing heattreatment may be utilized when one of the metal pieces comprises a lowalloy metal piece and the other metal piece comprises a stainless steelpiece. Such normalizing heat treatment may be utilized when one of themetal pieces comprises a low alloy ferritic steel piece and the othermetal piece comprises an austenitic stainless steel piece. Suchnormalizing heat treatment may also be utilized when one of the metalpieces comprises a martensitic stainless steel piece and the other metalpiece comprises either a of low alloy ferritic steel, austeniticstainless steel or martensitic stainless steel.

In another embodiment, the heat treatment comprises heating each metalpiece to a temperature that is sufficient to temper the HAZ to obtainadequate toughness, tensile and hardness properties in the HAZ. Forexample, in one embodiment such heat treatment comprises heating eachmetal piece above the A_(C1) transformation temperature, which variesdepending on the alloy and its corresponding chemistry, but below theA_(C3) temperature.

It should be appreciated that such tempering heat treatment may beutilized when one of the metal pieces comprises a low alloy metal pieceand the other metal piece comprises a stainless steel piece. Suchtempering heat treatment may be utilized when one of the metal piecescomprises a low alloy ferritic steel piece. For example, such temperingheat treatment may be utilized when one of the metal pieces comprises amartensitic stainless steel piece and the other metal piece comprises alow alloy ferritic steel. It should be appreciated that in some cases itmay be advantageous to use the normalizing heat treatment for one metalpiece and the tempering heat treatment for the other metal piece.

It should be appreciated that the advantage of heat treating each metalpiece before they are welded together avoids the need for PWHT.Therefore, in the case where the metal pieces comprise sub-components ofparticularly large component, heat-treating the individualsub-components may be easier than having to use PWHT for the entirefinished component or piece of equipment. As such, expensive field PHWTmay be avoided. Further, it should be appreciated that such heattreatment may be applied in the shop or in the field.

Additionally, after heat-treating each metal piece in the step 106, eachmetal piece may again be machined to the appropriate weld geometry inpreparation for final welding between the two metal pieces. Therefore,if such machining is performed, the thickness of the butter layer shouldbe sufficient to allow for a reduction in its thickness as a result ofsuch machining. The thickness of the butter layer may vary depending onwhich type of welding process, the filler composition and the alloycomposition is used.

In the next step 108, each of the heat-treated buttered metal pieces arewelded together using a nickel-based filler. As discussed above, thenickel base filler comprises at least 10% or more nickel by weight andmore preferably comprises approximately 40-70% nickel by weight, andeven more preferably 40-60% nickel by weight. Otherwise, the filler maybe any material known in the art, which will depend, in part, on thecomposition of the metal pieces being welded together. Examples of somepreferred filler materials capable of being used in the presentinvention for buttering and welding of dissimilar metals include INCONELWelding Electrode 182, INCONEL Filler Material 82 and INCO-WELD AElectrode. Preferably, the nickel-based filler used to weld the twometal pieces together is the same as the nickel-based filler used togenerate the buttered layers.

One of skill in the art will appreciate that any method or techniquesknown in the art may be used to weld the two metal pieces together.Similarly, any equipment known in the art may be used as well. It shouldbe appreciated that this welding operation would not require asubsequent PWHT as adequate properties have already been developed inthe respective HAZs in each of the metal pieces. Further, thenickel-based filler provides adequate metallurgical properties for thisweld. Therefore, the two welded metal pieces may be placed in servicewithout having to use PWHT.

FIGS. 2A-2C illustrate the welding of two metal pieces according to oneembodiment of the present invention. FIG. 2A illustrates two metalpieces 202, 204 to be welded. As noted above in connection with FIG. 1,each metal piece may be prepared according to preparation methods knownin the art, such as machining. FIG. 2B illustrates the application ofthe butter layers 206, 208 to each of the metal pieces. In thisparticular case, the butter layers 206, 208 are applied to the endsurfaces of each of the metal pieces 202, 204. As described above inconnection with FIG. 1, each of these metal pieces 202, 204 havingbutter layers 206, 208 would then be heat treated, either through anormalization heat treatment or a tempering heat treatment. In addition,each metal piece could be further processed by machining to achieve thedesired surface shape. FIG. 2C illustrates the results of the finalwelding step wherein a filler 210 is used to weld the butter layers 206,208, and, therefore, the two metal pieces 202, 204 together. Also asdescribed above in connection with FIG. 1, the final welded componentmay be placed in service without the need for PWHT.

Various embodiments of the invention have been described. Thedescriptions are intended to be illustrative of the present invention.It will be apparent to one of skill in the art that modifications may bemade to the invention as described without departing from the scope ofthe claims set out below. For example, it is to be understood that theinvention may be applied to welding of both similar and dissimilar metalpieces. Furthermore, it is to be understood that although the inventionhas been described generically for welding of two metal pieces, theinvention may be utilized in any application where metals are weldedtogether. For example, the invention may be utilized within the power,chemical, petroleum, steel, transportation, and pulp and paper. Moregenerally, any process wherein welding of a low alloy steel is employedmay make use of this technology. Additionally, the invention may be usedto weld a martensitic stainless steel to either a ferritic stainlesssteel, an austenitic stainless steel or another martensitic stainlesssteel.

1. A method for welding together two metal pieces, comprising: butteringa surface of a first metal piece with a first nickel-based filler metalat a thickness sufficient to isolate a heat-affected zone in the firstmetal piece from subsequent welding to produce a first buttered surface;heat-treating at least the heat-affected zone in the first metal pieceafter said buttering of the surface of the first metal piece to producea heat-treated first buttered surface; buttering a surface of a secondmetal piece with a second nickel-based filler metal having the samecomposition as the first nickel-based filler metal and at a thicknesssufficient to isolate a heat-affected zone in the second metal piecefrom subsequent welding to produce a second buttered surface;heat-treating at least the heat-affected zone in the second metal pieceafter said buttering of the surface of the second metal piece to producea heat-treated second buttered surface; and welding the heat-treatedfirst buttered surface to the heat-treated second buttered surface witha third nickel-based filler metal having the same composition as thefirst and second nickel-based filler metals.
 2. The method of claim 1,wherein the first, second and third nickel-based filler metals eachcomprise a nickel content of greater than 10% by weight.
 3. The methodof claim 2, wherein the nickel content is approximately 40-60% byweight.
 4. The method of claim 1, wherein each of said heat-treatingscomprise heat-treating at a temperature sufficient to normalize theheat-affected zone in each of the first and second metal pieces.
 5. Themethod of claim 4, wherein each of said heat-treatings each compriseheat-treating at a temperature above a corresponding Ac₃ temperature ofeach of said metal pieces.
 6. The method of claim 4, wherein the firstand second metal pieces each comprise a low alloy ferritic steel piece.7. The method of claim 4, wherein the first metal piece comprises a lowalloy metal piece and the second metal piece comprises a stainless steelpiece.
 8. The method of claim 7, wherein the first metal piece comprisesa low alloy ferritic steel piece and the second metal piece comprises anaustenitic stainless steel piece.
 9. The method of claim 4, wherein thefirst metal piece comprises a martensitic stainless steel piece and thesecond metal piece is selected from the group consisting of low alloyferritic steel, austenitic stainless steel and martensitic stainlesssteel.
 10. The method of claim 1, wherein each of said heat-treatingscomprise heat-treating at a temperature sufficient to temper theheat-affected zone in each of the first and second metal pieces.
 11. Themethod of claim 10, wherein each of said heat-treatings each compriseheat-treating at a temperature above a corresponding A_(C1) temperatureof each of said metal pieces and below a corresponding A_(C3)temperature of each of said metal pieces.
 12. The method of claim 11,wherein the first metal piece comprises a low alloy metal piece and thesecond metal piece comprises a stainless steel piece.
 13. The method ofclaim 12, wherein the first metal piece comprises a low alloy ferriticsteel piece and the second metal piece comprises an austenitic stainlesssteel piece.
 14. The method of claim 11, wherein the first metal piececomprises a martensitic stainless steel piece and the second metal pieceis selected from the group consisting of low alloy ferritic steel,austenitic stainless steel and martensitic stainless steel.
 15. Themethod of claim 1, wherein the first and second metal pieces eachcomprise a low alloy ferritic steel piece.
 16. The method of claim 1,wherein the first metal piece comprises a low alloy metal piece and thesecond metal piece comprises a stainless steel piece.
 17. The method ofclaim 16, wherein the first metal piece comprises a low alloy ferriticsteel piece and the second metal piece comprises an austenitic stainlesssteel piece.
 18. The method of claim 1, wherein the first metal piececomprises a martensitic stainless steel piece and the second metal pieceis selected from the group consisting of low alloy ferritic steel,austenitic stainless steel and martensitic stainless steel.
 19. A methodfor welding together two dissimilar metal pieces, comprising: butteringa surface of a first metal piece with a first nickel-based filler metalat a thickness sufficient to isolate a heat affected zone in said firstmetal piece from subsequent welding to produce a first buttered surface;heat-treating the first metal piece after said buttering of the surfaceof the first metal piece to produce a heat-treated first butteredsurface; buttering a surface of a second metal piece having a differentcomposition from the first metal piece with a second nickel-based fillermetal having the same composition as the first nickel-based filler metaland at a thickness sufficient to isolate a heat affected zone in saidsecond metal piece from subsequent welding to produce a second butteredsurface; heat-treating the second metal piece after said buttering ofthe surface of the second metal piece to produce a heat-treated secondbuttered surface; and welding the heat-treated first buttered surface tothe heat-treated second buttered surface with a third nickel-basedfiller metal having the same composition as the first and secondnickel-based filler metals.
 20. The method of claim 19, wherein thefirst metal piece comprises a low alloy metal piece and the second metalpiece comprises a stainless steel piece.
 21. The method of claim 20,wherein the first metal piece comprises a low alloy ferritic steel pieceand the second metal piece comprises an austenitic stainless steelpiece.
 22. The method of claim 19, wherein the first metal piececomprises a martensitic stainless steel piece and the second metal pieceis selected from the group consisting of low alloy ferritic steel,austenitic stainless steel and martensitic stainless steel.
 23. Themethod of claim 19, wherein each of said heat-treatings comprisesheat-treating at a temperature sufficient to normalize the heat-affectedzone in each of the first and second metal pieces.
 24. The method ofclaim 19, wherein each of said heat-treatings comprise heat-treating ata temperature sufficient to temper the heat-affected zone in each of thefirst and second metal pieces.
 25. A method for welding two dissimilarmetals, consisting essentially of: preparing a surface of a first metalpiece and a surface of a second metal piece having a differentcomposition from the first metal piece for welding; buttering thesurface of the first metal piece with a first nickel-based filler metalat a thickness sufficient to isolate a heat-affected zone in the firstmetal piece from subsequent welding to produce a first buttered surface;heat-treating at least the heat-affected zone of the first metal pieceto produce a heat-treated first buttered surface; machining the firstbuttered surface; buttering the surface of the second metal piece with asecond nickel-based filler metal having the same composition as thefirst nickel-based filler metal at a thickness sufficient to isolate aheat-affected zone in the second metal piece from subsequent welding, toproduce a second buttered surface; heat-treating at least theheat-affected zone of the second metal piece to produce a heat-treatedsecond buttered surface; machining the second buttered surface; andwelding the heat-treated first buttered surface to the heat-treatedsecond buttered surface with a third nickel-based filler metal having acomposition the same as the first and second nickel-based filler metals.26. The method of claim 25, wherein the first metal piece comprises alow alloy metal piece and the second metal piece comprises a stainlesssteel piece.
 27. The method of claim 26, wherein the first metal piececomprises a low alloy ferritic steel piece and the second metal piececomprises an austenitic steel piece.
 28. The method of claim 25, whereinthe first metal piece comprises a stainless martensitic steel piece andthe second metal piece is selected from the group consisting of lowalloy ferritic steel, austenitic stainless steel and martensiticstainless steel.
 29. The method of claim 25, wherein each of saidheat-treatings comprise heat-treating at a temperature sufficient tonormalize the heat-affected zone in each of the first and second metalpieces.
 30. The method of claim 25, wherein each of said heat-treatingscomprise heat-treating at a temperature sufficient to temper theheat-affected zone in each of the first and second metal pieces.